docs: def _apply_credentials(auto_refresh=True, credentials=None,
                             headers=None):
          """Update Authorization header.
  
          Update request headers with latest `access_token`. Perform token
          `refresh` if token is ``None``.
  
          Args:
              auto_refresh (bool): Perform token refresh if access_token is ``None`` or expired. Defaults to ``True``.
              credentials (class): Read-only credentials.
              headers (class): Requests `CaseInsensitiveDict`.
  
          """
          token = credentials.get_credentials().access_token
          if auto_refresh is True:
              if token is None:
                  token = credentials.refresh(
                      access_token=None, timeout=10)
              elif credentials.jwt_is_expired():
                  token = credentials.refresh(timeout=10)
          headers.update(
              {'Authorization': "Bearer {}".format(token)}
          )